The present invention relates to satellite communications, and more particularly, to satellite antenna pointing systems.
One of the primary uses of satellites is for communications. Commonly, a satellite will receive signals from transmitting stations located on the Earth, frequency translate and amplify these signals, then retransmit the signals to receiving stations located on the Earth. Satellites usually employ multiple antennas for the reception and transmission of signals for a variety of reasons. These reasons include: separating the transmitting and receiving functions and then providing multiple beams that communicate with different portions of the Earth, providing reuse of scarce frequency bands by using separate antennas that point in different directions while using the same frequency, and many others.
High-performance communications satellites use antennas that respond to signals from one direction much greater than from other directions. Hence, when using multiple antennas, each antenna must be pointed with meticulous accuracy to receive relatively weak communications signals from Earth based transmission stations or to transmit signals back to Earth based on receiving stations without overly degrading communications performance.
Conventionally, satellite antenna pointing systems use antennas that consist of an array of feeds that illuminate one or more reflectors to form beams. These antennas are positioned so that they can provide beams pointing towards a ground station on the earth""s surface.
A sensor is used to control the beams pointing directions. The sensor consists of the array of feeds, a tracking network that forms special beams called tracking beams and a tracking receiver. The sensor receives beacon signals transmitted from a station on the earth at a known pointing direction. The tracking receiver operates on the tracking beams to generate error signals that are proportional to the pointing error of the antenna. The error signals are used by the attitude control system to control a reflector positioning mechanism that steers the reflector relative to the satellite body to minimize pointing error.
A disadvantage of this arrangement is that each reflector must have a robust reflector positioning mechanism. The positioning mechanism must be robust so it will operate continuously over the typical 10 to 15 year lifetime of the satellite. Consequently, the reflector positioning mechanism is usually heavy and relatively costly to achieve this reliability.
A common goal in the design of satellites is to eliminate the cost and weight and to improve the reliability of all components including the reflector positioning mechanism. To achieve these goals some satellites have antennas mounted to a common thermally stable support structure.
Pointing of the antennas is done in response to a direction sensor connected to the support structure to estimate the pointing direction of the structure. The direction sensor is a separate antenna that, in conjunction with the tracking network, forms tracking beams. These are fed to the tracking receiver to form error signals, which are passed to the attitude control system of the spacecraft.
The attitude control system controls the satellite momentum wheels that in turn steer the entire spacecraft to minimize the pointing error seen by the antennas. One advantage of this system is that the reflectors can be deployed using a relatively simple reflector deployment actuator that must operate only once, at the time of the reflector deployment.
Unfortunately, this system also has several disadvantages. One disadvantage is that the support structure linking all reflectors and feeds must be very stable over temperature, and thus is costly to build. Another disadvantage is that the antennas must be built and integrated at the same time, making the integration process complex and time consuming.
Ultimately the desire is to eliminate the cost and weight of a reflector positioning mechanism while improving system reliability and to allow each antenna to be built independently and integrated at different times.
It is, therefore, an object of the invention to reduce weight and improve reliability by eliminating a reflector positioning mechanism. Another object of the invention is to reduce weight and improve reliability by eliminating a common thermally stable support structure.
In one aspect of the invention, a satellite antenna pointing system has a reflector antenna for receiving an uplink signal from an earth based ground station and a satellite based phased array assembly for transmitting a downlink signal. Because extraterrestrial communications suffer significant losses during transmission, accurate pointing of both uplink and downlink antennas is desired in order to minimize required signal strength. The present invention uses two different methods for pointing the uplink and downlink antennas.
The reflector antenna is used to receive uplink signals. The reflector antenna of the present invention is pointed to maximize reception of the uplink signal. A reflector pointing error sensor coupled to the reflector antenna to determine proper pointing direction. If the reflector pointing error sensor determines that the reflector antenna is not pointed properly, then a reflector adjusting device physically changes the pointing direction of the reflector antenna until the pointing error is minimized thereby maximizing the uplink signal strength.
The phased array assembly is used for transmitting the downlink signal. Because the phased array assembly is mounted in fixed relation to the reflector antenna, the phased array assembly is pointed after the reflector antenna has been successfully pointed. This is done using an array pointing error sensor attached to the phased array assembly to determine proper pointing direction. If the array pointing error sensor determines that the phased array assembly is not pointed properly, then a phased array controller electronically changes the pointing direction of the phased array assembly until the pointing error is minimized. Because the uplink and downlink antennas are pointed independently a common support structure is not necessary.
The present invention thus achieves a satellite antenna pointing system without the need for a reflector positioning mechanism or a common thermally stable support structure. This allows lower weight and manufacturing costs and has the added advantage of improving system reliability.
Additional advantages and features of the present invention will become apparent from the description that follows, and may be realized by means of the instrumentalities and combinations particularly pointed out in the appended claims, taken in conjunction with the accompanying drawings.